In the solvent-based coatings field wherein the composition that is applied for coating a substrate is a solution of a permanently thermoplastic vinyl copolymer in a non-reactive solvent that evaporates at the time of application, it has been a common practice to include small amounts of an acid monomer, such as maleic acid or acrylic acid in the mixture of monomers that is used to make the copolymer. See U.S. Pat. No. 3,037,955, example 1. This patent discloses that improvement in adhesion to bare metals is obtained when various free acids are added to the solution of the vinyl copolymer in small amounts of 1/2 to 3% by weight. U.S. Pat. No. 3,266,930 discloses the copolymerization of various vinyl and acrylic monomers with 0.5 to 10% by weight of an acid of the formula ##STR1## wherein R is H or CH.sub.3 and A is an alkylene group having 1 to 3 carbon atoms. The resulting copolymer is dissolved in a solvent, applied to a substrate, and dried and cured by a conventional baking operation to develop adhesion to metal substrates.
U.S. Pat. No. 3,300,547 discloses anaerobic adhesive sealant compositions comprising acrylate ester monomers, a hydroperoxide catalyst, and certain adhesion imparting materials (polymers, especially polyester resins, e.g., 80/20 styrene/acrylate polymer, having a molecular weight between 2,000 and 40,000) which may also contain an adhesive promoter "characterized by the presence of a polymerizable terminal double bond or ethylenic linkage adjacent a polar functional group of the compound", or a mixture of such promoters. Among the adhesive promoting materials, the patent mentions the following acids and their anhydrides: crotonic, itaconic, nadic, methylnadic, acrylic, methacrylic, and maleic.
U.S. Pat. No. 4,048,259 discloses the addition of an ester-acid of the formula: ##STR2## in which R is H or CH.sub.3, R' is H, halogen, -COOH, -CN, or a (C.sub.1 -C.sub.8) alkyl group and X is a hydrocarbon group of 1 to 6 carbon atoms, to an anaerobic adhesive comprising a polymerizable acrylic ester and an organic peroxide polymerization initiator therefor to improve the adhesion to metals, such as steel. This patent in col. 2 lines 12 to 17, acknowledges U.S. Pat. No. 3,300,547 with the comment "However, carboxyl substituents have a corrosive action toward steel".
100% solids polymerizable radiation-curable coatings are increasingly penetrating the metal finishing industry, especially in the metal decorating and can coating area. They are non-polluting and require markedly less energy for cure than conventional solvent based baking systems. However, virtually all radiation cured coatings based on vinyl unsaturation (primarily acrylic) exhibit adhesion inferior to baked coatings. It is generally agreed that a major factor contributing to poor adhesion is internal stress created by the rapid shrinkage on polymerization. With a baked coating, oven temperature is usually above the glass temperature allowing internal mobility for stress relaxation and conformance to the microstructure of the substrate. The post-baking of a radiation cured coating has been applied (U.S. Pat. No. 3,856,744) and it generally improves adhesion.
In U.S. Pat. No. 3,912,670, radiation-curable coating compositions are provided which contain a radiation curable oil, a flow control additive and at least 20% by weight of acrylic acid as reactive solvent, with or without other reactive solvent. The use of at least 20% by weight of acrylic acid in the composition is stated to give unexpectedly high adhesion to metals, especially to untreated aluminum. However, acrylic acid is quite volatile, has a strong acrid odor (like vinegar) and is a strong acid, highly corrosive to steel at these use levels.
In U.S. Pat. No. 4,052,280, a UV sensitizer is incorporated into the backbone of a UV-curable ethylenically unsaturated polymer in a particularly spaced relationship away from the ethylenic unsaturation, thereby improving adhesion to the substrate on radiation curing.
The reaction product of a hydroxyalkyl acrylate with a cyclic anhydride has been suggested as an adhesion-promoter in radiation-curable coatings (B. Trathnigg and J. Wendrinsky, Farbe and Lack, 82(2), 100 (1976).
In U.S. Pat. No. 3,943,103, the dissolution of a low molecular weight vinyl acetate polymer in a radiation-curable composition containing a polyfunctional reactive compound containing at least two radiation-curable polymerizable ##STR3## groups and optionally a monofunctional reactive compound having one such group is claimed to improve adhesion to metals.